1. Field of the Invention
The present invention relates to a photoresist and compounds of which the photoresist is made, and further relates to a method of forming a resist pattern by using the photoresist.
2. Description of the Related Art
In a field of manufacturing various electronic devices such as VLSI in which sub-micron order patterns are required to form, an electric device is now required to have higher densification and integration. Thus, an improved lithography technique for forming a more minute pattern on a substrate is required for satisfying such a requirement.
As one of methods for forming a more minute pattern is known a method in which an exposure light having a shorter wavelength is used for forming a resist pattern. For instance, there has been used i-line of high-pressure mercury vapor lamp, which has a wave length of 365 nm, as a light source for forming a dynamic random access memory (DRAM) having an integration of 64 M bits or less. In a mass production of a 0.25 .mu.m-rule 256 M bits DRAM, it is now being expected that the i-line is replaced with KrF excimer laser, as an exposure light source, having a shorter wavelength than that of the i-line, more specifically, having a wavelength of 248 nm. However, the fabrication of 0.18 .mu.m-rule 1 G bits or greater DRAM which requires a technique for forming a more fine patterns needs a light source having a shorter wavelength than that of KrF excimer laser to be developed. For that purpose, an ArF excimer laser having a wavelength of 193 nm now attracts attention as a light source to be used in photolithography. For instance, the utilization of ArF excimer laser is discussed by T. Ueno, T. Iwayanagi, K. Nono, H. Ito and C. Grant Willson: "Resist Materials for Short Wavelength" Bunshin Shuppan Inc., 1988.
Hence, there is now expected to develop a new resist to be employed for photolithography in which ArF excimer laser is to be used. Such a resist used for ArF light exposure is required to enhance cost performance of laser, because a gas from which laser is radiated has short life-time, and also because a laser radiating equipment is expensive. Thus, the resist is expected to have a high resolution as well as a high sensitivity in response to a design rule getting smaller and smaller.
As one of methods of enhancing a sensitivity of resist, there has been well known a chemically amplified resist which utilizes a photo acid generator as a sensitizer. For typical instance, Japanese Patent Publication No. 2-27660 has suggested a resist comprised of a combination of triphenylsulfonium hexafluoroarsenate and poly (p-tert-butoxycarbonyloxy-.alpha.-methylstyrene). There are many reports about a chemically amplified resist for use with a KrF excimer laser, for instance, one of which is American Chemical Society Symposium Series, 1984, Vol. 242, pages 11-23, reported by Hiroshi Ito and C. Grant Willson. A chemically amplified resist is characterized by the steps of generating proton acid by exposing a photo acid generator to a light, and transferring the thus generated proton acid through a resist solid phase by means of post-exposure heat treatment to thereby amplify chemical reaction of the resist resin up to hundreds of to thousands of times due to the proton acid in a way like catalytic action. Thus, a quite higher sensitivity can be attained relative to a prior resist which has an optical reaction efficiency, which is defined as a reaction per a photon, of smaller than one (1).
Presently, most of newly developed resists are chemically amplified resists, and hence a chemical amplification system has to be adopted in development of a high sensitive material in response to a wavelength of an exposure light source getting smaller and smaller.
In a conventional lithography in which there is employed an exposure light having a longer wavelength than that of KrF excimer laser having a wavelength of 248 nm, a photoresist contains a resin having aromatic rings in a structural unit, such as novolac resin and poly (p-vinylphenol). The dry etching resistance of such aromatic rings gives a dry etching resistance to the above mentioned resin.
However, the aromatic rings quite intensively absorb lights having a wavelength equal to or shorter than 220 nm such as ArF excimer laser having a wavelength of 193 nm, and hence the above mentioned conventional resins cannot be applied to a photolithography in which there is used a light source emitting deep ultraviolet radiation (DUV) having a wavelength equal to or shorter than 220 nm. Thus, there is now studied a resin which does not contain aromatic rings, but has etching resistance, and which contains alicyclic hydrocarbon. For instance, such a resin has been reported by S. Takechi, Journal of Photopolymer Science and Technology, Vol. 5, No. 3, 1992, pp. 439-446.
However, since alicyclic groups have strong hydrophobic property, the introduction of alicyclic groups causes a resin containing the alicyclic groups to have stronger hydrophobic property, which would cause problems that adhesion of a formed thin film to a silicon substrate is deteriorated, and that the uniformity of a thickness of a film to be formed on a substrate is also deteriorated.
As one of solutions to the problems, there has been suggested the use of methacrylic acid unit. For instance, the inventors have already suggested the copolymer in "Positive Chemically Amplified Resist for ArF Excimer Laser Lithography Composed of a Novel Transparent Photoacid Generator and an Alicyclic Terpolymer", Proceeding of SPIE, Vol. 2438, 1995, pp. 433-444. The copolymer consists of tricyclodecanylacrylate, tetrahydropyranylmethacrylate and methacrylic acid, and is a resin used for a resist into which alicyclic groups and methacrylic acid units are introduced.
Thus, a polymer having transparency to a light having a wavelength of 193 nm and also having a dry-etching resistance can be attained by introduction of alicyclic polymers, and the above mentioned problems of deterioration in adhesion and film thickness uniformity which would be caused by the introduction of alicyclic polymers can be overcome by introduction of methacrylic acid units. The above mentioned resin for composing a resist generally includes tetrahydropyranyl groups and tert-butyl groups as a protective group which is decomposed by acid and changes a polarity of a polymer. For instance, the use of a tetrahydropyranyl group has been reported in K. Nakano, K. Maeda, S. Iwasa, T. Ohfuji and E. Hasegawa: "Positive Chemically Amplified Resist for ArF Excimer Laser Lithography Composed of a Novel Transparent Photoacid Generator and an Alicyclic Terpolymer", Proceeding of SPIE, Vol. 2438, pp.433-444, and the use of a tert-butyl group has been reported in R. D. Allen, G. M. Wallraff, R. A. Dipietro, D. C. Hofer and R. R. Kunz: "Single Layer Resists with Enhanced Etch Resistance for 193 nm Lithography", Journal of Photopolymer Science and Technology, Vol. 7, No. 3, 1994, pp. 507-516.
However, a tetrahydropyranyl group has shortcomings of a low thermal decomposition point and low thermal stability. In addition, it is known that a tetrahydropyranyl group produces a polymer as a result of side reaction, when used as a protective group for polyvinylphenol, as reported in T. Sakamizu, H. Shiraishi, H. Yamaguchi, T. Ueno and N. Hayashi: "Acid-Catalyzed Reactions of Tetrahydropyranyl-Protected Polyvinylphenol in a Novolak-Resin-Based Positive Resist", Japanese Journal Application Physics, Vol. 31, 1992, pp. 4288-4293.
Similarly, when a tetrahydropyranyl group is used as a protective group for methacrylic acid, there will be formed a polymer as a result of side reaction in a way as shown in the following chemical reaction formula [5]. Namely, a tetrahydropyranyl group has faults as a protective group that it will produce a polymer as a by-product which prevents the resist from being dissolved to a developing agent to thereby deteriorate resolution of the resist, and further thereby produce resist residue or scum. ##STR2##
On the other hand, a tert-butyl group does not proceed desorption quantitatively unless a strong acid such as triphlate acid is used. Hence, a photo acid generator to be used in the photoresist is limited to one which would produce triphlate acid, such as triphenylsulfoniumtrifluoromethanesulfonate. However, triphlate acid has high volatility, and tends to be evapotraspired from a resist film before a resist is developed. In addition, as triphlate acid is a strong acid, it readily reacts with basic compounds existing in the air. Thus, there tends to be produced a so-called surface dissolution resisting layer on which, when triphlate acid is used, triphlate acid is deactivated and thus cannot reach an effective amount at a part of a surface of a resist film, and hence a pattern on the part cannot resolved. As a result, the resolution of a resist is remarkably deteriorated. The study about this phenomenon is provided by S. A. MacDonald et al., Proceedings of SPIE, 1991, Vol. 1446, pp. 2-12.
As discussed above, a resist for lithography employing a light having a wavelength equal to or smaller than 220 nm has the etching resistance and adhesion to a substrate, but does not have protective groups for changing polarity of a resin. Thus, there has been suggested no resists having high resolution and high sensitivity by which a fine pattern can be obtained without production of scum.